1. Field of the Invention
This invention relates to a dual motor system for driving a photoreceptor belt with a balanced torque to improve image registration in an electrophographic electrophotographic imaging system.
2. Brief Description of Related Developments
Electrophotographic printing machines employ photoreceptor members, typically in the form of a belt that is electrostatically charged to a potential so as to sensitize the surface thereof. The charged portion of the belt is exposed to a light image of an original document being reproduced. Exposure of the charged member selectively dissipates the charge thereon in the irradiated areas to record an electrostatic latent image corresponding to the informational areas contained within an original document. After the electrostatic latent image is recorded on the photoreceptor member, a developer material is brought into contact therewith to develop the latent image. The electrostatic latent image may be developed using a dry developer material comprising carrier granules having toner particles adhering triboelectrically thereto or using a liquid developer material. Toner particles are attracted to the latent image, forming a visible powder image on the surface of the photoreceptor belt. After the electrostatic latent image is developed with the toner particles, the toner powder image is transferred to a substrate, such as a sheet of paper. Thereafter, the toner image is heated to permanently fuse the image to the substrate.
In order to reproduce a color image, the printing machine includes a plurality of imaging stations each of which deposits a toner of a given color. Each station has a charging device for charging the photoreceptor surface, an exposing device for selectively illuminating the charged portions of the photoreceptor surface to record an electrostatic latent image thereon, and a developer unit for developing the electrostatic latent image with toner particles. Each developer unit deposits different color toner particles on the electrostatic latent image. The images are developed, at least partially, in superimposed registration with one another to form a multi-color toner powder image. The resultant multi-color powder image is subsequently transferred to a substrate. The transferred multi-color image is then permanently fused to the sheet forming the color print. To obtain a high quality color image, registration of the images at each of the developer stations is essential.
Registration is achieved by accurately positioning the photoreceptor belt at the various imaging and developing stations along the belt path using a drive mechanism that typically comprises drive rollers that advance a substrate along the path and backer bars that support the belt. Many such drive rollers have a coating commercially known as an EPDM elastomer that is applied to the surface thereof to improve friction coupling between the drive mechanism and the belt. Due to backer bar and subsystem drag, the drive rollers often experience slippage at the photoreceptor belt and at other locations along the belt when the surface of the drive roller encounters particle contamination. Slippage has a deleterious impact on image registration, particularly when latent images are applied at multiple imaging stations.
An auxiliary belt drive may address slippage problems, but in order to be effective, the torque level and proper location of the auxiliary drive is essential to attain optimum drive benefit while at the same time satisfying motion quality and registration requirements of the imaging system. In addition, belt tensioning and drive capacity requirements must also be met.
One solution to the slippage problem is presented in U.S. Pat. No. 6,421,523 which issued to the same assignee as this application. This patent describes a belt drive module that achieves the above goal by providing a torque assist drive that applies a torque assist force to the belt at a location between the drive roller and the tension roller. In this instance the torque assist force is provided by a constant torque friction clutch or a current limited DC motor. This system operates in a torque limiting manner.
Image registration may be more difficult in designs where low friction between the drive roll and the belt occurs due to a large wrap angle. In these situations dual drive rolls are needed to apply the required torque to the photoreceptor belt. It a purpose of this invention to provide a dual roll drive mechanism for a photoreceptor belt. It is also a purpose of this invention to distribute the torque between the drive rolls in a predetermined manner to maintain a constant torque on the belt.